The infrastructure for cellular telephone systems, as well as for other mobile communication systems, includes antennas that are typically mounted on towers. The antenna receives radio signals from the cellular telephone, The received radio signal is then transmitted down a transmission line to a base station that is located next to the antenna. The base station then processes the signal and routes the signal to its proper destination such as another cellular telephone or to a telephone company for connection with a wire-line telephone.
The radio signals that are transmitted from the cellular telephones can be very weak. Accordingly, the range of the base-station antenna is limited and the telephone must be fairly close to receive a signal strong having sufficient strength. One way to increase the range of a base-station antenna is to increase the power output by the cellular telephone. However, this method raises health concerns because the antenna for hand-held cellular telephones is placed adjacent to a user's head and brain.
Another way to increase the range of a base-station antenna coverage is to increase its sensitivity. One way to increase a base-stations antenna's sensitivity is to place an amplifier at the top of the tower so that the signal received by the base-station antenna is amplified before it is transmitted along the transmission line to the base station. The amplification increases the strength of the signal and overcomes any loss that occurs as the signal is transmitted from the antenna to the base station. This amplification permits the antenna and base station to effectively receive and process relatively weak signals. As result, the sensitivity and coverage range of the antenna is effectively increased.
When an antenna is mounted in a remote location such as high on a tower, it is advantageous to be able to remotely test it. Remote testing is accomplished by transmitting a signal from the base station to the antenna. If a signal having a certain strength is reflected from the antenna back to the base station, there is an error. For example, this reflected signal may indicate that there is an open circuit in the antenna or that the antenna leaked and is full of water. The ability to remotely test antennas is especially important for several reasons. For example, it permits the antenna to be frequently and periodically tested, which helps to maintain a high level or reliability of the cellular infrastructure. Another reason is that climbing towers, which can be as high as 400 feet or more is dangerous, especially in winter or other adverse weather.
However, an amplifier in the line prevents the test signal from reaching the antenna because of the reverse isolation of the amplifier and thus it cannot be tested. This problem is a serious impediment to being able to use a tower-top amplifier to increase the sensitivity of the antenna. Even if the signal could reach the antenna, the amplifier would present a practical problem because it would amplify the test signal as it is reflected to the base station. As a result, the amplitude of the reflected test signal received at the base station always would be above a certain threshold level, which could falsely indicate a failure in the antenna.
Therefore, there is a need for a system that enables testing of a remote device that has its output amplified. There is a further need for a system for bypassing a portion of the output circuit for a remote device. There is also a need for a system that neutralizes any gain or amplification of a signal that is output from a remote device.